Rhodopsin, the visual pigment present in the rod photoreceptors of many vertebrates, is centrally involved in the processs of visual adaptation within the retina. As yet, the mechanisms by which visual pigment influences receptor sensitivity are not well understood. The overall objectives of the project are to elucidate (1) the dependence of rod photoresponses on the content of rhodopsin; (2) the visual activity of artificial, opsin-containing pigments; and (3) the receptoral reactions of vitamin A during dark adaptation. The principal type of experiment utilizes the isolated, all-rod retina of the skate, and involves the extracellular measurement of the aspartate-isolated, massed photoreceptor potenial. Receptor thresholds and maximal response amplitudes will be compared under conditions of brief, intense irradiation, versus sustained, relatively dim illumination. The physiological effects of 11-cis retinal (the chromophore of rhodopsin) and synthetic chromophores will be examined following their application to light-adapted photoreceptors. For the investigation of receptoral vitamin A reactions during dark adaptation, putative precursors of 11-cis retinal will be applied to strongly light-adapted rabbit photoreceptors, and the ability of the receptors to transform these compounds into 11-cis retinal will be monitored electrophysiologically and spectrophotometrically.